This trait provides a limited interface, only allowing reading of elements.
There are two extensions of this trait, in packages
scala.collection.mutable
and
scala.collection.immutable, which provide functionality for
adding new key/value mappings to a map. The trait in the first package is
for maps that are modified destructively, whereas the trait in
the second package is for immutable maps which create a new map
when something is added or removed from them.